314 PHYSICAL FEATURES OF THE UNITED STATES 



waste was continued, and so the eastern part of the Atlantic belt 

 is what geographers call a constructional plain. But there is 

 another part, lying close to the mountains, which shared in the 

 Appalachian uplift and also in the Appalachian carving, and was 

 finally reduced so nearly to sea level that it constitutes an in- 

 separable part of the Atlantic Province. It consists of ancient 

 rocks, graded down nearly to a uniform level, and is clast by 

 geographers as a destructional or eroded plain. As Pluto raises 

 and lowers the land the ocean is caused to alternately recede and 

 advance, and this low-lying plain is peculiarly susceptible to its 

 encroachment. In our day the fourth part of it is submerged, 

 so that its actual limit as a physical feature lies many miles be- 

 yond .the coast, where there is an abrupt change from shallow 

 soundings to abyssal depths. The land of the Atlantic Plain is 

 shaped for agriculture, and much of it is cultivated ; but there 

 are broad tracts of soil too poor to compete with the fertile land 

 of the Central Plain and utilized only for timber and other forest 

 products. Water powers, afforded by the moderate fall of large 

 streams, have great value by reason of their proximity to tide- 

 water and consequent facilities for cheap transportation of the 

 raw materials and the products of manufacture. 



The Lake Province, overlapping all other provinces from the 

 north, is a marginal overflow of Canadian topography, and re- 

 sulted from the great prehistoric invasion of our land by Cana- 

 dian ice. The colossal ice-sheets of the eastern and central 

 British provinces and the contemporary glaciers of the northern 

 Cordilleran mountains remodeled the topography of all the prov- 

 inces, carving the valleys into new shapes and heaping the debris 

 in irregular mounds and ridges of peculiar type. When the ice 

 was melted and rains fell again upon the land, the streams could 

 neither find nor follow their old courses, and the waters were 

 compelled to fill many a hollow before the}^ could flow away at 

 all; so while the old types of mountains and plains remained as 

 broad features characterizing the several provinces, there was 

 added the feature of obstructed drainage, markt by a multi- 

 plicity of lakes. Of these are the lakes and ponds of New Eng- 

 land and New York, the great Laurentian lakes and their host of 

 associated lakelets, the mountain lakes of Idaho and Montana, 

 and the curious linear lakes of northern Washington. The dis- 

 tribution of ores was not affected, though facility of discovery 

 and exploitation was locally modified, being partly impaired 

 and partly improved. The surface conditions bearing on agri- 



